Underwater detection system

ABSTRACT

An anti-torpedo torpedo having laser homing head, the torpedo being launched from a parent vessel or platform which itself houses the expensive to replace, the bulky, or the high power consuming parts of the laser, said parts being optically connected to an optical projector in the homing head in the torpedo itself and to any parts of the laser not housed in the parent vessel.

This invention relates to underwater detection systems suitable for usein weapons systems capable of intercepting an approaching torpedo. Theweapon systems include an underwater weapon, and a parent vessel orplatform, the system having guidance means including a laser, preferablyemitting light in the blue/green spectral bands. Underwater laser lightis rapidly scattered and becomes incoherent; this limits the range atwhich a target can be detected to a few hundred meters depending uponthe output powers of the laser, but assuming that the weapon can bebrought to within that range, then a laser guidance system of typehaving light projection means and light detection means to detect thereflection from a target can be utilised.

Several prime advantages accrue from such a laser guidance system.

-   -   1. Because the speed of light is about 2.10⁸ m/s in water the        delay due to projection to the target and the reflection back        (the propagation delay) is insignificant, and the rate at which        data can be acquired is limited only by the laser pulse rate.    -   2. Since the propagation delay is of the order of nanoseconds or        microseconds even a freely rolling weapon will not move        significantly in this period. There is no requirement for weapon        roll position control and the weapon is thus simplified.    -   3. An imaging detector can be utilised to improve countermeasure        resistance and target characterisation.    -   4. The system is likely to be relatively unaffected by        hydrodynamically induced noise and can allow relatively high        speed operation.

According to the present invention, an underwater detection systemhaving an underwater guided vehicle, a parent vessel (either a surfaceor an underwater vessel) or platform and detection means for detectingunderwater objects, the detecting means including a laser lightgenerating means, means to direct the light so generated towards anunderwater object, and detector means to receive the reflection fromsaid underwater object, at least some parts of the laser lightgenerating means being carried by the parent vessel or platform and atleast the directing means and the detector means being carried by theunderwater vehicle, those parts of the laser light generating meanscarried by the parent vessel or platform being operatively connected tothe directing means and to any parts of the laser light generating meanscarried by the underwater vehicle by optical fibre cable means.

By this arrangement any expensive, bulky or high power consuming partsof the laser light generating means can be mounted in the parent vesselthereby reducing the size and cost of replacement of the underwatervehicle.

Typically laser which could be used in the light generating means are afrequency doubled Nd-yag laser (532 nm), a mercury bromide laser (502nm) or a Raman shifted XeCI laser (500 nm).

Advantageously, in the case of the Nd-yag laser, which has a basicwavelength of 1.06 um, if carried by the parent vessel can have itsfrequency doubling crystal carried in the underwater vehicle. Thisallows the light transmitted by the optical fibre to be in or close tothe lowest loss band of current optical fibres which occurs at the nearinfra red wavelength.

In the case of the Raman shifted XeCI laser, the material of the opticalfibre may be selected so that it acts as a Raman convertor for thelaser.

In a further embodiment it is suggested that, for example in the case ofNd-yag laser, both the yag crystal (i.e. the laser rod) and thefrequency doubling crystal could by carried by the underwater vehiclewhilst the high power absorbing pulsed lighting used for stimulating theyag crystal into resonation is carried by the parent vehicle, and thepulses passed to the crystal via the optical fibre.

One embodiment of the invention as applied to an anti-torpedo underwaterweapon system is described by way of example with reference to theaccompanying drawing which is a diagrammatic representation of thesystem.

An underwater vehicle, in this case a weapon is illustrated at 10. Itcomprises a generally cylindrical body with a transparent window II inthe nose behind which lies an optical projector 12 and and imagingsystem 13. The choice of guidance enables the weapon to operate withoutroll control; it needs therefore only fixed stabilising fins 14 togetherwith a single nozzle rocket motor 15 for propulsive power. Directionalcontrol is by thrust vector control e.g. by swivelling the nozzle of themotor 15.

The underwater vehicle 10 is associated with a parent vessel or platform16 shown only in broken outline by way of information. A fibre opticcable 17 extends between the vehicle 10 and the vessel 16, eachpreferably having a dispenser reel 18, 19 respectively from which thecable 17 unwinds as the vehicle 10 moves away from the vessel 16.

Laser light generating equipment is carried in the vessel 16. It isconnected via an optical coupler, the fibre optic cable 17 to an opticalprojector 12, which may be merely the polished end face of the opticalcable 17 suitably located within the nose region of the vehicle 10.

The optical cable 17 extends along the centre line of the vehicle 10 andterminates just aft of the transparency 11 in the polished end facewhich forms the optical projector 12.

In use the laser light generator in the parent vessel 16 provides aseries of intense, coherent, light pulses via the optical cable 17 tothe projector 12 and thence to an object for example a torpedo, whichrequires to be intercepted. Any variations in backscatter caused by thepresence of a torpedo are detected by the imaging system 13 whichincludes a lens, a gated intensifier, and an electronics package whichdeciphers the image received and passes steering correction signals tothe thrust vector control system (not shown). These items areconveniently mounted coaxially with the optical fibre table 17.

To provide scanning of the target area in one or more planes, anoscillating optical wedge is positioned in the optical path between thelens and transparency.

The described embodiment envisages all the equipment associated with thelaser light generator 20 to be carried in the parent vessel 16. Howeveras previously discussed there is advantage in certain circumstances forsome of this equipment (e.g. frequency doubling crystals or YAGcrystals) to be removed from the vessel 16 and placed in the underwatervehicle 10 for example at reference 20 a in the figure.

1. An underwater detection system having an underwater guided vehicle, aparent vessel which can be a surface vessel or an underwater vessel or aplatform, and detection means for detecting underwater objects, thedetection means including a laser light generating means, means todirect the light so generated towards an underwater object, and detectormeans to receive a reflection from said underwater object, at least someparts of the laser light generating means being carried by the parentvessel and at least the directing means and the detector means beingcarried by the underwater vehicle, and optical fiber cable means foroperatively connecting those parts of the laser light generating meanscarried by the parent vessel or platform to the directing means and toany parts of the laser light generating means carried by the underwatervehicle.
 2. An underwater detection system comprising: an underwaterguided vehicle having an optical projector for directing received laserlight towards an underwater target, and an optical detector forreceiving laser light reflected from the target; a laser lightgenerator, remote from said underwater guided vehicle; and a light guidefor transmitting laser light from said generator to said guided vehicleand coupling said laser light to said guided vehicle as said receivedlaser light.
 3. An underwater detection system according to claim 2 inwhich said laser light generator is incorporated in a parent vessel. 4.An underwater detection system according to claim 3 in which said laserlight generator is a Nd-YAG laser, and further comprising a frequencydoubler incorporated within said guided vehicle for doubling a frequencyof light transmitted from said generator via said light guide.
 5. Anunderwater detection system according to claim 2 which said light guideis a fibre optic cable.
 6. An underwater detection system according toclaim 5 in which the optical projector is a polished end face of saidfibre optic cable located in a nose portion of said guided vehicle. 7.An underwater detection system according to claim 2 in which said laserlight generator is a pulsed laser light generator.
 8. An underwaterdetection system according to claim 2 in which said optical detectorincorporates a lens, gated intensifier and electronic means forgenerating a steering correction signal for the guided vehicle inresponse to received laser light reflected from the target.
 9. Anunderwater detection system according to claim 8 in which the opticaldetector further incorporates an oscillating optical wedge for scanningof the target.